In communication systems and methods, messages or data from a sender to a receiver using a communication link are sometimes subject to sending errors, such as bit errors, delay, reordering, and duplication. For example, noise on the communication link can cause bits within messages to be incorrect, generally causing the receiver to be unable to use the message. In a wireless communication system, these problems are increased by circumstances such as co-channel interference, multipath and multipoint effects, such as refraction or reflection resulting in intrasymbol interference and intersymbol interference. These problems can substantially reduce the reliability of wireless communication links.
Automatic retransmission protocols are conventionally used between sender and receiver, so that the receiver positively acknowledges (ACK) or negatively acknowledges (NAK) messages from the sender, and the sender re-transmits those messages not acknowledged by the receiver within a reasonable time. Automatic retransmission and error recovery protocols, such as Automatic Repeat reQuest (ARQ) and hybrid-ARQ (i.e. ARQ with Forward Error Correction (FEC)), are known for packet-based communication.
The constantly varying quality of many wireless communication channels makes ARQ a highly desirable element of wireless data transfer mechanisms. Some attempts have been made to provide ARQ functionality in wireless multicast traffic delivery. Mil-Std-188-184 requires that each forward transmission be followed by a round-robin series of ACK/NAK transmissions from the receiving stations, transmitted one recipient at a time. The P-MUL (ACP142) protocol defers transmission of ACKs or NAKs until the entire traffic payload has been transmitted. Then, each recipient transmits a succession of ACKs and NAKs of the packets contained in the transmission. No explicit provision to prevent collisions of ACKs and NAKs transmitted by different recipients is included; the P-MUL protocol specification merely suggests delaying each ACK transmission by a randomly-determined period of time.
Users have found the multicast capability of MIL-Std-188-184 very slow and inefficient; as a result, it is not commonly used. The inefficient acknowledgment procedure for P-MUL and the lack of adaptability of transmission characteristics to channel conditions place limits on the level of performance this technique can achieve.
The NATO standard STANAG 4538 describes a pair of data link protocols HDL (High throughput Data Link) and LDL (Low latency Data Link), used for efficient point-to-point multipoint (multicast or broadcast) data transfer. Performance of the HDL and LDL protocols is enhanced through their use of a hybrid-ARQ (incremental redundancy) technique, in which if a packet is received with errors, the received channel symbols from receipt of the packet are retained; the receiving station requests retransmission of the corrupted packet; if the retransmitted packet still contains errors its symbols are combined with those retained from the prior transmission; as a result of the combining of symbols, it is often possible to recover the transmitted packet without errors.
U.S. Patent Application No. 2003/0227934 to White et al. describes a conventional multicast ARQ scheme for ad hoc networks where retransmissions can be multicast or unicast. This approach is similar to the Mil-Std-188-184 Multicast ARQ scheme. U.S. Pat. No. 6,151,696 to Miller, et al. describes a NAK-only multicast data transfer with ARQ.
U.S. Patent Application No. 2003/0012195 to Ohkubo et al. is directed to a multicasting system, in which the base station judges whether a received signal indicates a retransmission request according to the quality of the received signal, and retransmits a multicast signal corresponding to the retransmission request based thereon. In addition, the base station monitors a receiving state of a multicast signal in the mobile stations, and changes a transmission method to conform to the receiving state according to a result of monitoring, and sends a multicast signal. Only negative acknowledgements (NAKs) are used, and NAKs from multiple recipients can coincide or overlap in time through use of spread NAK waveform. Different recipients transmit different NAKs using different spreading codes, so that recipient identity can be recovered (e.g. from NAK received with highest power and/or quality).
As a result of increases in High Frequency (HF) network communications capacity, use of HF for delivery of IP network traffic is becoming useful and practical for many users. A major unmet challenge is that of providing an effective capability for delivering multicast data traffic (from one sender to multiple recipients) over HF channels. Lack of such capabilities is frequently an obstacle to using HF as a bearer medium for certain applications. Accordingly, there is a need for an efficient multicast data transmission protocol over wireless media such as HF radio.